This is my first post by the way.
First off - I love this site. Great info and workouts. Keep up the good work!
And I really like the nutritional input.

BTT: I have been experimenting a bit and I find you can really boost the overall effectiveness of the SimpleFit program by swapping the body weight squats for two-handed Kettlebell swings (36 pounds or heavier). Apart from that keep the workout as is.

Sure, it is no longer a body weight only workout, but you will get GREAT results because the kettlebell will improve your grip strength by far and it will also really boost middle and lower back development. It will also dramatically increase the cardio component and boost endurance beyond measurement.

Give it a try and let me know how you like it.
Do it for one or two weeks though at least.

I also incorporate the KB into my workouts. I have a 1 and 1.5 pood ball. However, in my opinion, bodyweight squats wear me out more than KB swings. So I try to do both. I'll do the Simplefit workout, mark the time, and then time how long it takes to do 100 swings, or something like that. I wouldn't feel right not doing my squats.